1. Field of the Invention
The present invention is related to a transmission speed predicting apparatus and a transmission speed predicting method, and more particularly, is applicable to a communication terminal of a digital type portable telephone system, and also to a transmission speed predicting method in this communication terminal.
2. Description of the Related Art
Recently, as a wireless connection system between a base station and communication terminals, which constitute a digital type portable telephone system, there is such a wireless communication system called the code division multiple access (CDMA) system.
In the CDMA system, one transfer speed (hereinafter, referred to as "wireless transfer speed") of data used in wireless sections between a base station and communication terminals is defined as, e.g., 19,200 bps. Also, another transfer speed (hereinafter, referred to as "transmission processing speed") used in a processing executed within the base station when data is transmitted (hereinafter, referred to as "transmission processing") is defined to be 1/n ("n" being an arbitrary integer value) of the wireless transfer speed, e.g., four types of 9,600 bps, 4,800 bps, 2,400 bps, and 1,200 bps in order that this transmission processing speed is switched in response to line qualities of the base station and data qualities used therein.
On the other hand, in a communication terminal of a portable telephone system to which such a CDMA system has been applied, every time the transmission processing speed of 9,600 bps used in a transmission processing is successively lowered to such lower transfer speeds of 4,800 bps, 2,400 bps, and 1,200 bps, amounts of data produced by these lower transmission processing speeds are successively reduced. As a result, in the communication terminals, such data produced at the transmission processing speeds lower than 9,600 bps is repeatedly transmitted plural times previously set by an arbitrary number in such a manner that the data amounts produced by these lower transmission processing speeds are apparently equal to the data amount produced by the transmission processing speed of 9,600 bps. In other words, the communication terminals execute the different transmission processing from each other every time the transmission processing speed under use is changed.
As a consequence, for example, when one communication terminal receives data transmitted via the base station from another communication terminal, the communication terminal is required to process this received data (hereinafter, referred to as "reception processing") at the same transfer speed (hereinafter, referred to as "reception processing speed") as the transmission processing speed of the received data. However, normally, in the case that data is transmitted from a communication terminal, the communication terminal transmits this data without adding information about a transmission processing speed used in the communication terminal.
As a result, the communication terminal may execute the four types of reception processing using the same reception processing speeds as the above-described four types of transmission processing speeds with respect to the received data. Also, the communication terminal may predict a transmission processing speed of the received data based upon the various sorts of information acquired by these reception processing, and then may select data acquired by the reception processing using the reception processing speed substantially equal to the predicted transmission processing speed among the four types of data acquired in the four types of reception processing.
In an actual communication terminal, a cyclic redundancy check (CRC) code is added as an error detection code to a data to be transmitted in order that when the data is received, the error detection of this received data is executed, and then based on the result of this error correction, a transmission processing speed may be predicted. However, in general, the CRC code is added only to such data produced at the relatively higher transmission processing speeds, i.e., 9,600 bps and 4,800 bps. As a result, in the case that such data produced at the transmission processing speeds of 2,400 bps and 1,200 bps is received by a communication terminal, there is a problem that this communication terminal can hardly predict the transmission processing speeds of 2,400 bps and 1,200 bps.
Also, in a communication terminal, data is convolution-coded to be transmitted while a transmission processing is carried out. Every time this received data is processed by the four types of reception processing, the Viterbi-decoding operations are performed. Then, this communication terminal may predict a transmission processing speed based upon the most likelihood pathmetric values (namely, values expressed by numerically processing certainties of data series obtained by decoding) of data series acquired during the Viterbi-decoding operations. As to each of the four types of reception processing, this communication terminal compares polarity information corresponding to the data obtained before being Viterbi-decoded with polarity information corresponding to such data which is obtained by again convolution-coding the Viterbi-decoded data to thereby acquire the incoincident number of the polarity information ("1", or "0"). Thus, this communication terminal may predict this incoincident number of the polarity information as the number of errors occurred in the Viterbi-decoded data, and then may predict a transmission processing speed based on this error number (hereinafter, referred to as "predicted error number").
However, when a relatively large number of errors are produced in data decoded by the Viterbi-decoding operation in communication terminals, there are two possibilities. That is, the most likelihood pathmetric values acquired by executing the four types of reception processing are substantially equal to each other. Furthermore, the predicted error numbers acquired by executing the four types of reception processing are substantially equal to each other. In these possible cases, it is difficult to predict the transmission processing speed correctly.